As a heat transfer system, a system of pressing and heating by means of a thermal head has so far been put to practical use, and it has excellent features in the aspects of noiselessness, simple mechanism, maintenance free and dry processing. Furthermore, with regard to image resolution, the density of a thermal head has been enhanced and thereby resolving power of 400-6000 DPI has recently been attained. However, it is difficult to expect further enhancement of density for a thermal head. Thus, there has been a limit of resolving power in conventional heat transfer recording systems.
With the background mentioned above, there has been suggested a laser heat transfer system employing a laser beam as a heat source. In this system, a laser beam can be condensed to about a few microns. Therefore, resolving power can be enhanced remarkably. However, due to high image fineness, image transfer onto a transfer sheet whose surface is highly irregular or wavy is not satisfactory. That is, image resolving power is greatly influenced by how a transfer sheet can be brought into close contact with a sheet.
With regard to contact of a sheet with the surface for exposure in the past, there have been considered some methods wherein a sheet is wound round a drum and both ends of the sheet are pulled to strech it around the drum, or air between the sheet and the drum is sucked to the inside of the drum for vacuum attraction. However, the methods mentioned above alone make it difficult to obtain sufficient close contact between an ink sheet and a transfer sheet. Therefore, sufficient transferability has not been obtained. Further, in order to obtain sufficient vacuum attraction, a method to add fine grains having a particle size of several microns called a matting agent to an image-receiving layer or to an ink layer can be considered. In this method, however, the surface is matted and thereby the close contact is inhibited, resulting in uneven transferring, which has been a problem.
For solving the problem mentioned above, it is effective to cause a transfer sheet or a transfer medium to have cushionlike characteristics for improving the contact condition. However, the method mentioned above is not a fundamental solving means for the so-called rough sheet with the surface having low smoothness because vacuum attraction does not give sufficient pressure.
Therefore, there may be considered a method wherein an image is transferred temporarily onto an intermediate image-receiving sheet having a smooth image-receiving surface and the image is further re-transferred onto an objective image-receiving sheet by means of lamination or the like. In this case, however, a method to transfer an image with an image-receiving layer is relatively easy, but it has a drawback of deterioration of an image grade such as creation of a gloss because of existence of an image-receiving layer on a non-image area on the final image.
The inventors of the present invention found, after studying a method for transferring only an ink image to an objective transfer sheet, that it is possible to transfer an image onto an intermediate image-receiving sheet with sufficient sensitivity by employing tackifer such as rosin in an ink layer and employing a certain kind of thermoplastic elastomer in the intermediate image-receiving sheet and thereby to re-transfer an ink image onto a printing sheet or the like by pressing and heating an image on the intermediate image-receiving sheet.
However, the method mentioned above requires high pressure of at least 20 kg/cm.sup.2 or more, and in an ordinary laminator, there occurs dispersion in transferring and ink is transferred from the intermediate image-receiving sheet through cohesive failure, thus, it is difficult to control image density, which is another problem. Therefore, it is desired that almost all ink can be transferred onto a sheet at a laminator pressure of about 2-3 kg/cm.sup.2 without deteriorating transfer sensitivity for the intermediate image-receiving sheet.
Methods wherein an image is recorded on an intermediate image-receiving sheet and then the image is pressure-transferred onto any given image-receiving sheet such as a rough sheet or the like are disclosed in Japanese Patent Publication Open to Public Inspection Nos. 127334/1979, 161946/1979, 295094/1986 and 128987/1988 (hereinafter referred to as Japanese Patent O.P.I. Publication). In these methods, however, it has been difficult to form an image with high resolution in the order of several microns corresponding to laser recording, or to re-transfer the image.